This invention relates to the treatment of gas streams and is more particularly concerned with the removal of pollutants from waste gases associated with industrial and utility processes. Within its general scope of applicability, the invention is especially useful, for example, in the removal of sulfur compounds (e.g., gaseous SO.sub.2) from waste gases generated by the combustion of sulfurbearing fossil fuels in boilers and the like.
Since the establishment of government standards restricting atmospheric emissions from stationary sources, industry has continually strived to devise effective emission control technologies which are both economical and operationally practical. In the case of certain classes of emissions, numerous technical problems have made this task a particularly onerous one which engineers have managed to meet with only limited success. Gaseous SO.sub.2 emissions from industrial and utility boilers, which have been regulated for over 10 years, are exemplary in this regard. With respect to utility installations, for instance, flue gas desulfurization still remains a serious financial obstacle to the construction of new coal-fired facilities and the conversion of existing oil-fired facilities to coal, despite the considerable effort which has been expended in attempts to solve this problem.
Nearly all flue gas desulfurization schemes proposed heretofore have been based upon a chemical reaction between the SO.sub.2 constituent of the flue gas stream and a sorbent material (usually an alkali) with which the gas stream is contacted to form a solid or water soluble residue which can be collected for disposal. Due largely to cost considerations, sodium and calcium-based sorbent materials have been the most widely applied in practice. As it is generally recognized that the reaction of SO.sub.2 with sodium and calcium-based sorbents proceeds more quickly when both are dispersed in water, wet flue gas scrubbing systems have dominated industry practice to date. However, while wet scrubbers are indeed capable of adequate SO.sub.2 control, they are nonetheless subject to significant disadvantages. For example, the fluid and residue handling requirements of wet scrubbers generally necessitate overall systems which are quite complex as well as impractical for utility-scale application. Thus, efforts have continued toward the development of practical alternatives.
In a broad sense, dry sorbent injection systems represent perhaps the most attractive alternative to wet scrubbers among previously known flue gas desulfurization techniques. The potential operational and cost advantages of this approach (which basically involves the injection of finely divided dry sorbent material directly into a flue gas stream) are readily apparent, as the need for water or the like is eliminated while advantage can still be taken of less expensive sorbent materials. Unfortunately, attempts to implement dry sorbent injection technology have, by and large, proved disappointing--primarily due to physical limitations on reaction kinetics. Recent attempts at dry injection in conjunction with baghouse dust collectors have provided some encouraging results, but this approach still faces significant technical problems and has yet to be proven commercially feasible.